This invention relates to the thermal processing of metallurgical alloys, and, more particularly, to the heat treating of gamma titanlum-aluminide alloys.
Titanium aluminides are a class of alloys whose compositions include at least titanium and aluminum, and typically some additional alloying elements such as chromium, niobium, vanadium, tantalum, manganese, or boron. The gamma titanium aluminides are based on the gamma phase field found at nearly the equiatomic composition, with roughly 50 atomic percent each of titanium and aluminum, or a slightly reduced aluminum content to permit the use of other alloying elements. The titanium aluminides, and particularly the gamma titanlum-aluminide alloys, have the advantages of low density, good low and Intermediate temperature strength and cyclic deformation resistance, and good environmental resistance.
Gamma titanium aluminides have application In aircraft engines. They can potentially be used in applications such as low-pressure turbine blades and vanes, bearing supports, compressor casings, high pressure and low pressure hangars, frames, and low pressure turbine brush seal supports.
One area of continuing concern in the titanium aluminides, and particularly the gamma titanium aluminides, is their low-to-moderate levels of ductility. Ductility is the measure of how much a material can elongate before it fails, and is linked to other properties such as fracture resistance. The gamma titanium-aluminide alloys typically elongate at most only 1-4 percent prior to failure, and have a steeply rising stress-strain curve. Maintaining the strength and resistance of the material to premature failure is therefore highly dependent upon controlling the alloy ductility.
Gamma titanium aluminides are typically prepared by melting, casting, hot isostatic pressing to reduce the porosity resulting from the casting, and thereafter heat treating to achieve an acceptable ductility level. It has been found from experience that the preferred combination of hot isostatic pressing and heat treating temperatures for optimum ductility depends upon the aluminum content of the alloy. That is, different processing procedures have been developed for gamma titanium-aluminide alloys of different aluminum contents. Even then, however, the aluminum content is sometimes difficult to control and measure with the accuracy required in the selection of the preferred processing.
One solution to this problem has been to use a combination of a moderate hot isostatic pressing temperature of 2200.degree. F. followed by a high heat treating temperature of 2375.degree. F. that produces reasonably good ductility properties for a wide range of aluminum contents. Unfortunately, the high heat treating temperature In this processing requires a specialized furnace that is expensive and may not be economically available in all instances.
There is a need for an improved processing procedure for gamma titanium-aluminide alloys to attain good properties using readily available and economic processing facilities. The present invention fulfills this need, and further provides related advantages.